I built a solar engine a while ago. The circuit is really fascinating how it gathers electricity and stores it until it reaches a point sufficient enough to power a heavier load than its source input. I found this circuit the other day and thought I would share it (plus it will help me remember where it is if I post it here).
Below is the solar engine schematic:
Here's the link to the page: Solar Engine
This gentleman also has a lot of other schematics. I haven't looked through all of them, but there are some pretty interesting circuits.
Here's his main link: Here
On my spare time I'm going to look through these and build one. I'll post the creation later!